The field of the invention is that of blending dispensers for liquids. More particularly, the field is that of blending dispensers capable of dispensing a variety of blending mixes.
In liquid dispensing devices, for example gasoline fuel pumps, different grades of gasolines are conventionally mixed to provide a specific blend. Using two gasoline sources, one supply tank containing gasoline having a first octane rating, for instance, 95, and the other supply tank containing gasoline having another octane rating, for instance, 83, a dispenser can produce gasoline blends having a desired octane rating in the range of 83 to 95. The following example uses 83 and 95 octane gasolines as the component fuels because they represent a common prior art range of octanes; however, a variety of ranges of octanes have been used to mix and provide blends of gasoline. For example, gasolines of various octane ratings can be provided by mixing component gasolines in the following percentages:
______________________________________ Octane % of 83 octane % of 95 octane ______________________________________ 83 100 0 87 67 33 89 50 50 91 33 67 95 0 100 ______________________________________
It is imperative in such blending dispensers that the component fuels are accurately dispensed to provide the desired blend ratio.
In prior art systems, a proportional control system has been used to check the blending ratio periodically by observing the instantaneous flow rates of the two component streams of fluid (e.g. from the 83 and 95 octane tanks). An electronic controller observes the instantaneous flow rates by receiving signals from pressure sensors coupled to the two flow lines. When the instantaneous flow rates substantially deviate from those needed to provide the desired mixture, the valves which control the component streams are adjusted accordingly. This results in a dispenser which tends to deliver an instantaneous blend ratio equal to the desired mixture only after a significant portion of the desired quantity of fuel has been dispensed, assuming that liquid pressure remains relatively constant. Thus a significant quantity of the dispensed fuel may have an incorrect blend or mixture. However, such proportional control systems only correct for errors in the instantaneous flow rate and do not account for the total errors in fuel which has already been dispensed so that the total amount of dispensed fuel will be a desired blend.
Consider a hypothetical system having 83 octane and 95 octane gasoline which uses a proportional control system to provide a range of gasolines having different cumulative blend ratios (i.e., the amount of 83 octane dispensed divided by the sum of the amount of 83 and 95 octane dispensed). Initially, relatively little of the 83 octane is dispensed compared to the amount of 95 octane dispensed. After periodic checking and correcting of the instantaneous blend ratio, the cumulative blend ratio curve steadies as the dispenser eventually provides an instantaneous blend of approximately 50% of 83 octane fuel and 50% of 95 octane fuel. However, the initial disparity does not enter into the calculation of the instantaneous rates, so that the total quantity of dispensed fuel for the session contains less than the desired 50% of 83 octane fuel. Accounting for the total blend ratio, a steady state error exists created by the initial mixture which contained substantially more 95 octane fuel than 83 octane fuel. The steady state error represents that amount of 95 octane gasoline which exceeds the amount of 83 octane gasoline, and the error can never be eliminated because the instantaneous rates are maintained at the 50% ratio. The cumulative blend ratio does approach the desired cumulative ratio as the amount of fuel dispensed approaches infinity, but the average consumer does not dispense such a large volume of fuel, rather the typical volume of gasoline dispensed by a consumer is between five and ten gallons. Thus, as a practical matter, the steady state error does not substantially decrease for gasoline dispensing applications because of the low volume of dispensing. Additionally, small quantities of dispensed fuel tend to have large errors. Also, random fluctuations of pressure within the proportional control system can add to the steady state error.
For example, assume that an operator or customer desired to put 10 gallons of 89 octane gasoline into the automobile fuel tank. Initially, for some reason, the 95 octane gasoline flowed at a rate four times that of the 83 octane gasoline. After the first gallon, the dispenser had provided 0.2 gallons of 83 octane gasoline and 0.8 gallons of 95 octane gasoline. At the one gallon mark, the dispenser corrected the difference in flow rates, and the remainder of the ten gallons were dispensed in an approximately 50% blend ratio. Thus at the end of the dispensing cycle, the dispenser provided 5.3 gallons of 95 octane gasoline compared to 4.7 gallons of 83 octane gasoline, resulting in a cumulative blend ratio of 4.7/(4.7+5.3) or 47%.
In the same example, assume that immediately after five gallons had been dispensed, the pressure in the 83 octane gasoline line fell dramatically so that only 0.3 gallons were dispensed while 0.7 gallons of 95 octane gas were dispensed.
After that dramatic pressure drop, the system then corrected for the pressure differential and dispensed an approximately 50% blend ratio. Thus, at the end of this dispensing, the dispenser provided 5.5 gallons of 95 octane gasoline compared to 4.5 gallons of 83 octane gasoline, resulting in a cumulative blend ratio of 4.5/(4.5+5.5) or 45%.
One form of prior art dispenser uses fixed orifice valves with pressure equalizers to achieve proper blending, generally having a cumulative accuracy of approximately 3%. The size of the orifice for each gasoline determines the instantaneous blend because the pressures are maintained relatively equal by pressure equalizers. However, to provide a blend which does not correspond to specific orifice sizes requires that the valves of the dispenser must be periodically turned on and off. Because the pressure sensors, valves, and pressure equalizers of dispensers are least accurate during starting and stopping, using fixed orifice blenders limits the versatility of the blending dispensers.
In dispensing operations having more than one dispenser for each pair of tanks, which is the usual circumstance, pressure equalizers often cannot perform adequately because of the variable pressures from the supplying tanks. For example, if three dispensers are currently dispensing 95 octane gasoline, and then a fourth dispenser is set to provide 89 octane gasoline, the fluid pressure in the supply line from the 95 octane supply tank should be substantially less than the pressure from the 83 octane supply tank. The resulting imbalance between pressures causes imprecision in accurately providing the desired cumulative blend.
Also, those dispensers tabulate the total amount dispensed, not the individual amounts dispensed from each tank. This can present difficulty to sellers of gasoline who must accurately measure the amount of gasoline withdrawn from each supply tank. Although an estimate of the individual amounts withdrawn are made from the total amount dispensed, additional proving must occur to verify the estimated amounts. Further, to insure that the desired cumulative blend is produced so the octane rating is at least as high as desired, the dispenser is biased in favor of dispensing more of the higher octane, more expensive gasoline.
Another feature of gasoline dispensers is that the operator may choose to dispense the gasoline quickly or slowly by actuating the nozzle handle. Thus, the operator can control the nozzle of the dispenser to change the flow rate or turn the flow on or off at any time. Due to the unpredictability of the operation of the nozzle, the signals sent by the pressure sensors may differ greatly at different times. When the nozzle is off, no flow exists, and the periodic signals sent by the pressure sensors may not present adequate information to make an accurate blend calculation. Thus, prior art dispensers have further problems due to erratic and unpredictable use which further impairs their accuracy.
Thus, a need exists for an improved blending dispenser that avoids the above-mentioned problems and accurately provides a desired cumulative blend. Also needed is a dispenser which separately measures the volume of the component fluids mixed and dispensed. A further need exists for a blending dispenser which adjusts component fluid flows according to the total cumulative volume of flow rather than instantaneous flow rates.